International Journal of Engineering Trends and Technology (IJETT) – Volume 22 Number 1- April 2015
Voltage flickering mitigation using D-Statcom
Anoop B K#1, Binil Kumar*2, Manoj K C#3
#
Assistant Professor,ECE,Vimal jyothi engineering college kannur
Abstract— Power quality in distribution systems has been
attracting an increasing interest during recent years.
Research studies include the quality of voltage supply with
respect to temporary interruptions, voltage dips,
harmonics, and voltage flicker. Voltage flicker occurs
when large industrial loads, such as electric arc furnaces,
rolling mills, and pumps operate periodically in a weak
power distribution system. The most commonly used
device for compensation of voltage flicker is the Static Var
Compensator (SVC).
This paper presents the digital modelling
and simulation of the voltage flicker phenomenon, which
was observed in a 3 phase 415 V Utility/Customer
distribution system. The simulation tool is the
MATLAB/Simulink Power System Block set (PSB). The
simulation shows that the degree of voltage flicker at the
PCC exceeds the permissible flicker limits. To mitigate the
voltage flicker problem of this distribution system, a 15
Mvar current control PWM-based DSATCOM is added to
the location of the flicker source in this PSB model.
Keywords— Voltage
improvement.
flickering,
D-Statcom,Power
Quality
I.
INTRODUCTION
Power quality in distribution systems has been attracting
an increasing interest
during recent years. Research
studies include the quality of voltage supply with respect to
temporary interruptions, volta ge dips, harmonics and
voltage flicker . Voltage flicker occurs when large
industrial loads, such as electric arc furnaces, rolling
mills, and pumps operate periodically in a weak power
distribution system. It causes voltage fluctuation at the
Point of Common Coupling (PCC) with other loads and
can annoy residential consumers by causing visible
lighting flicker on incandescent or fluorescent lamps
Problems with electricity supply may always occur
regardless of time and place. This may cause an impact to
the electric supply thus may affect the manufacturing industry
and impede the economic development in a country. The
major electric problems that always occur in power systems
are the power quality problems that have been discussed by
the electrical engineers around the world, since problems have
become a major issue due to the rapid development of
sophisticated and sensitive equipment in the manufacturing
and production industries
The increased concern for power quality has resulted in
measuring power quality variations, studying
the
characteristics of power disturbances and providing
solutions to the power quality problems. In distribution
systems, the power quality problems can reduce the
power supplied to the customers from its nominal value.
Voltage sag, harmonic, transient, overvoltage and under
voltage are major impacts to a distribution system. The utility
ISSN: 2231-5381
and the users are responsible in polluting the supply network
due to operating large loads
There are many solutions to mitigating the power
quality problems at a distribution system such as using surge
arresters, active power filters, isolation transformer,
uninterruptible power supply and static VAR compensator.
Blazicet al. proposed a new D-STATCOM control algorithm
which enables separate control of positive and negative
sequence currents and decoupled control of d- and q- axes
current components. From the studies, it is shown that all
these equipment’s are capable to solve power quality
problems. The best equipment to solve this problem at
distribution systems at minimum cost is by using Custom
Power family of D-STATCOM
Environmental effects also give an impact to the
power quality and its reliability. Major concerns on
industrial p o w e r quality problems are that they affect the
production, due to sensitive equipment in the industries.
Where there are power qualities problems, equipment may
mis-operate or machine may possibly shut down.
Installations by industries such as Adjustable Speed Drive
(ASD), switch mode power supplies and high frequency
switching also affect the power quality. High sensitivity
equipment such as high speed motor, super computer,
microprocessors and medical instruments may also be
affected by the power quality problems occurring in the
system
The D- STATCOM has emerged as a promising device to
provide not only for voltage sags mitigation but a host of
other power quality solutions such as voltage stabilization,
flicker suppression, power factor correction and harmonic
control. The D-STATCOM has additional capability to
sustain reactive current at low voltage, reduce place required.
So it can be developed as a voltage and frequency support by
replacing capacitors with batteries as energy storage. In
addition to the perceptible and sometimes irritating
lighting flicker to humans, voltage flicker can also
cause electrical equipment efficiency drop, torque and
power oscillations,
and
interference
in
protection
systems. Modern consumers require high quality power
supply for their sensitive facilities. Voltage flicker h a s
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International Journal of Engineering Trends and Technology (IJETT) – Volume 22 Number 1- April 2015
therefore been a n important power quality concern for both
power companies and customers
II. VOLTAGE FLICKERING
Residential customers near large industrial plants often
experience flickering lights. This voltage flickering can be
extremely harmful to sensitive electronic equipment.
Computerized equipment requires stable voltage to perform
properly. For this reason, voltage flicker is a major power
quality problem. Flicker is a difficult problem to quantify and
to solve. The untimely combination of the following factors is
required for flicker to be a problem: 1) some deviation in
voltage supplying lighting circuits and 2) a person being
present to view the possible change in light intensity due to
the voltage deviation. The human factor significantly
complicates the issue and for this reason flicker has
historically been deemed "a problem of perception." The
voltage deviations involved are often much less than the
thresholds of susceptibility for electrical equipment, so major
operating problems are only experienced in rare cases. To
office personnel, on the other hand, voltage deviations on the
order of a few tenths of one percent could produce extremely
annoying fluctuations in the output of lights, especially if the
frequency of repetitive deviations is 5-15 Hz
Due to the clear relationship between voltage deviation and
light response, the term "flicker" often means different things
to different people with the interpretations. In each case, the
deviation may or may not be strictly periodic and is usually
expressed as a change (as indicated by the change in rms
value) relative to the steady-state level (expressed as an rms
value averaged over some time period). For voltage variations,
the change is usually expressed as DV/V. A similar expression
for light intensity variations also exists
The primary cause of voltage changes is the time
variability of the reactive power component of fluctuating
loads. Such loads include arc furnaces, rolling mill drives, and
mine winders — all of which are loads with a high rate of
change of power with respect to the short-circuit capacity at
the point of common coupling (PCC).
However these approaches are quite cumbersome and
expensive. The mechanical switches and relays are sluggish,
unreliable, require frequent maintenance and introduce
switching transients.
A 3phase synchronous motor when over excited works as a
synchronous condenser or a capacitor. It gives dynamic power
factor correction over a wide range of its excitation. When
under excitation, it operates at a lagging power factor and
therefore absorbs reactive power from the bus. When over
excited, a synchronous motor works at leading power factor
and therefore acts as a generator of reactive power and
therefore behaves as a capacitor. A static capacitor bank
provides power factor control in discrete steps whereas a
synchronous condenser furnishes a continuous control of
power factor improvement and the associated reactive power
flow. A synchronous condenser has more losses and it is very
slow as compared to a capacitor bank and also it can be
installed at one place only. These are the disadvantages of
synchronous condenser
Static var compensator consists of a thyristor controlled
reactor (TCR) in parallel with a fixed capacitor C. Thyristor
controlled reactor is a major component of a Static VAR
Compensator. Static thyristor controlled reactors are
connected in parallel with the load for the control of reactive
power flow. With increase in size of industrial connected
loads fast reactive power compensation has become necessary
Small power loads, such as starting of induction motors,
welders, boilers, power regulators, electric saws and hammers,
pumps and compressors, cranes and elevators also can be
sources of flicker. Other causes are capacitor switching and
on-load transformer tap changers, which can change the
inductive component of the source impedance. Variations in
generation capacity of wind turbines, for example, also can
have an effect. Sometimes, voltage fluctuations are caused by
low-frequency voltage inter-harmonics.
III. METHOD USED FOR VOLTAGE FLICKERING MITIGATION
A bank of capacitors is connected across the load.
Since the capacitor takes leading reactive power, over all
reactive power taken from the source decreases, consequently
system power factor improves.
In this method capacitance across the motor terminals
must be varied as the load on the induction motor alters. Thus
a continuous control of power factor would entail the lead of a
large number of capacitors of small rating. The switching in or
out is carried out by means of relays and circuit breakers.
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III. D-STATCOM
The D-STATCOM is a three-phase and shunt
connected power electronics based device. It i s connected
near the load at the distribution systems. The major
components of a D-STATCOM are shown in Fig It consists
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International Journal of Engineering Trends and Technology (IJETT) – Volume 22 Number 1- April 2015
of a DC capacitor, three-phase inverter (IGBT, thyristor)
module, AC filter, coupling transformer and a control strategy.
𝑉𝐷𝐶 =
3√3.𝑉𝑠. 𝑐𝑜𝑠𝛼
𝜋
(3)
where,
α = delay angle
if α = 0, the equation becomes,
𝑉𝐷𝐶 =
Inverter Operation
Inverters are used to convert DC signal to AC signal. In
this work a 3-phase inverter has been developed. The DC
source in the system is the DC capacitor. This is located in
parallel with the D- STATCOM.
𝜋
(4)
The value of VCMAX is the present upper limit of
CDC and is two or three times of the VDC.
The heavy load is connected to the PCC
through a circuit breaker. It is designed in such a way that it is
initially open and get closed at 0.2sec and sustain its action.
Due to this heavy load there occurs a voltage sag in the PCC
and nearby consumers suffer a voltage flicker.
This voltage dip is mitigated by using DSTATCOM connected at the PCC. The circuit breaker
connected to the STATCOM is designed in such a way that it
is initially open and transition occurs at 0 and 0.4 sec and
open after 0.4 sec. Hence the STATCOM works upto 0.4sec.
Initially IGBT works as a converter and produce a DC output
voltage, which charges the capacitor to a high value with the
help of series inductance.
The voltage across the capacitor terminal is
measured and it is given to a numerical divider. The Vpc gets
divided with a constant value set as 1000. This is fed to a PI
controller named as Vpc regulator. The output of the
controller is the correction for compensating the active power
consumed by the STATCOM. The load current Iabc is
converted to dq0 frame and it is extracted to d,q,and 0
componentby using a demultiplexer .
Capacitor Operation
Capacitor sizing is referred to the fault
current in the system. The difference in current between
the current before and after the fault is considered as current
faults. In capacitor sizing, a suitable range of DC capacitor
is needed to store the energy to mitigate the voltage sag.
The DC capacitor, CDC is used to inject reactive power to
the D-STATCOM when the voltage is in sag condition. In
the design, the harmonic effects must be considered because
the load is inductive and this may affect the value of CDC.
The following equation is used to calculate CDC
½ CDC[VCMAX2-VDC2] = ½ VSM.∆IL.T
(1) is used for harmonic mitigation in
single phase system but for a three phase system the
equation is given by,
3𝑉𝑠 ∆𝐼𝐿 𝑇
𝐶𝐷𝐶 =
(2)
𝑉
𝑉
𝐶𝑀𝐴𝑋2 − 𝐷𝐶2
where,
VS = peak phase period of one cycle of voltage and
current
VCMAX = pre-set upper limit of the energy storage C
(per-phase),
VDC = voltage across C (per-phase).
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3√3.𝑉𝑠.
2
(𝑉 sin(𝜔𝑡) + 𝑉𝑏 sin(𝜔𝑡 − 2𝜋/3) + 𝑉𝑐 sin(𝜔𝑡 + 2𝜋/3)
3 𝑎
𝑉𝑑 =
𝑉𝑞 =
2
3
𝑉0 =
(𝑉𝑎 cos(𝜔𝑡) + 𝑉𝑏 cos(𝜔𝑡 − 2𝜋/3) + 𝑉𝑐 cos(𝜔𝑡 + 2𝜋/3)
1
(𝑉𝑎 + 𝑉𝑏 + 𝑉𝑐 )
(5)
3
for used for the conversion of Va,Vb,Vc to Vd,Vq V0
The correction Iloss is added with the d component
of the load current.The per unit value of the load voltage is
compared with Vref , a constant value using the PCC voltage
regulator. It is also a PI controller and the output will be the
correction in the q component of the load current. This
correction Iqr is added with the q component.
Instantaneous phase of the voltage is obtained from a
three-phase PLL. The extracted dq and 0 components are then
given to a multiplexer. The output of the multiplexer is in dq0
frame. It is then converted into abc frame. The output of dq0
to abc frame is compared with load current Iabc. This output
will be an error signal. This signal is fed to the SPWM.
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International Journal of Engineering Trends and Technology (IJETT) – Volume 22 Number 1- April 2015
IV. SIMULATION RESULT
Fig 3Without D-STATCOM with load
Fig 2 without D-STATCOM without load
(A)Stabilisation Of DC Link Voltages
(B)stabilisation In The Voltage at PCC
Fig (A) shows the stabilization of dc link
voltages. The capacitor initially charged to 1200V and gets
stabilized at 1060V.
Fig (B) shows the voltage profile at load 2 and load 3
side. The STATCOM is on from 0 sec to 0.4 sec and the
heavy load2 is switched on only at 0.2 sec and another load 3
is switched on at 0.3 The voltage dip at the starting is due to
capacitor charging. At 0.2 sec the heavy load starts working. It
draws more reactive current and STATCOM supplies the
required amount. At 0.3 sec third load starts working. It
draws much more reactive current and STATCOM supplies
the required amount. At 0.4 sec the STATCOM is turned off
and no more reactive power is supplied by STATCOM. Thus
the entire reactive power required by the load need to be given
by the source itself, which leads to more losses which in turn
results in further voltage dip.
With D-STATCOM Without Load
Fig 2 gives the output of without STATCOM
without load. It is an ideal condition. Fig 3 gives the output of
real condition what happens when a load is powered on. It is
without STATCOM and with load. Fig 4 resembles the
condition when a STATCOM presents and no load. It’s the
condition before .2 seconds of the simulation
V. CONCLUSION
In the present situation where even the power system is
going to be privatized, a number of companies are moving
towards the production and distribution of electric power. So
power quality and its improving devices becomes important,
so in this era D-STATCOM shall become popular
A detailed model of D-STATCOM has been developed for
using simulink environment with the power system block set.
Models of both power circuit and control system have been
implemented in the same simulink diagram allowing smooth
simulation. The matlab simulation results show that the fast
response and flexible control of the D-STATCOM allow for
an efficient voltage flicker mitigation in distribution system
Here by using D-STATCOM in 415 voltage three phase ac
source along with a heavy load of 5kW, it is shown that
voltage dip can be improved by 30V and the overall power
quality of the system gets improved
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International Journal of Engineering Trends and Technology (IJETT) – Volume 22 Number 1- April 2015
REFERENCE
[1].
J. Sun, D. Czarkowski and Z. Zabar, "Voltage
Flicker Mitigation Using PWM-Based Distribution
STATCOM", Power Engineering Society Summer
Meeting, 2002 IEEE Volume 1, Issue , 25-25 July
2002 Page(s):616 - 621 vol.1
[2].
Modeling and Simulation of a D-STATCOM using
Simulink’s Power System Blocket,27th annual
conference of IEEE industrial Electronics Society
[3].
“FACTS controllers in power transmission and
distribution”, K. R.Padiyar. First edition New Age
international publishers , 2007.
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